Imaging device including photoelectric conversion layer

ABSTRACT

An imaging device including pixels having a photoelectric converter including a first and second electrode, a photoelectric conversion layer; a charge accumulation region electrically connected to the first electrode; and a signal detection circuit. The photoelectric converter is applied with a voltage between the first electrode and the second electrode, and the photoelectric converter has a characteristic, responsive to the voltage within a range from a first voltage to a second voltage, showing that a density of current passing between the first electrode and the second electrode when light is incident on the photoelectric conversion layer becomes substantially equal to that when no light is incident on the photoelectric conversion layer. A difference between the first voltage and the second voltage is 0.5 V or more, and the voltage supply circuit supplies a voltage between the first voltage and the second voltage to the second electrode in a non-exposure period.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/444,841, filed on Nov. 11, 2016, which is a continuation of U.S.patent application Ser. No. 15/963,410, filed on Apr. 26, 2018, now U.S.Pat. No. 10,375,329, which is a continuation U.S. patent applicationSer. No. 15/719,456, filed on Sep. 28, 2017, now U.S. Pat. No.9,986,182, which is a continuation of International Application No.PCT/JP2016/004868, filed Nov. 11, 2016, which in turn claims the benefitof Japanese Application No. 2015-236858, filed on Dec. 3, 2015, thedisclosures of which are incorporated in their entirety by referenceherein.

BACKGROUND 1. Technical Field

The present invention relates to an imaging device.

2. Description of the Related Art

Heretofore, image sensors using photoelectric conversion have beenknown. For example, complementary metal oxide semiconductor (CMOS) typeimage sensors that have photodiodes are in widespread used. CMOS typeimage sensors have features such as low power consumption, andaccessibility to individual pixels. CMOS type image sensors generallyuse the so-called rolling shutter method, where exposure and signalcharge readout is performed in increments of rows of the pixel array, asthe signal readout method.

In rolling shutter operations, the starting time and ending time ofexposure differs for each pixel array row. Accordingly, in a case ofshooting an object moving at high speed, a distorted image may beobtained as the image of the object, and when using a flash, there maybe difference in brightness throughout the image. In light of thissituation, there is demand for so-called global shutter functions, whereall pixels in the pixel array start and end exposure together.

For example, U.S. Patent Application Publication No. 2007/0013798discloses a CMOS type image sensor capable of global shutter operations.The technology described therein provides a transfer transistor and acharge storage unit (a capacitor or a diode) to each of multiple pixels.The charge storage unit in each pixel is connected to a photodiode viathe transfer transistor.

SUMMARY

One non-limiting and exemplary embodiment provides an imaging devicecapable of realizing global shutter functions while suppressing circuitcomplexity within pixels.

In one general aspect, the techniques disclosed here feature an imagingdevice including pixels each having a photoelectric converter includinga first electrode, a second electrode, a photoelectric conversion layerbetween the first electrode and the second electrode; a chargeaccumulation region electrically connected to the first electrode; and asignal detection circuit electrically connected to the chargeaccumulation region. The imaging device further includes a voltagesupply circuit electrically connected to the second electrode. Thephotoelectric converter is adapted to be applied with a voltage betweenthe first electrode and the second electrode, and the photoelectricconverter has a characteristic, responsive to the voltage within a rangefrom a first voltage to a second voltage, showing that a density ofcurrent passing between the first electrode and the second electrodewhen light is incident on the photoelectric conversion layer becomessubstantially equal to that when no light is incident on thephotoelectric conversion layer. In addition, a difference between thefirst voltage and the second voltage is 0.5 V or more, and the voltagesupply circuit supplies a voltage between the first voltage and thesecond voltage to the second electrode in a non-exposure period.

General or specific embodiments may be implemented as an element, adevice, an apparatus, a system, an integrated circuit, a method, or acomputer program. General or specific embodiments may also beimplemented as any selective combination of an element, a device, anapparatus, a system, an integrated circuit, a method, and a computerprogram.

According to an aspect of the present disclosure, global shutterfunctions can be realized while suppressing circuit complexity withinpixels.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary circuitconfiguration of an imaging device according to an embodiment of thepresent disclosure;

FIG. 2 is a schematic cross-sectional view illustrating an exemplarydevice structure of a unit pixel cell;

FIG. 3 is a diagram illustrating an example of an absorption spectrum ina photoelectric conversion layer containing tin naphthalocyanine;

FIG. 4 is a schematic cross-sectional view illustrating an example ofthe configuration of a photoelectric conversion layer;

FIG. 5 is a graph illustrating typical photocurrent characteristics thatthe photoelectric conversion layer has;

FIG. 6 is a diagram for describing an example of operations in animaging device according to an embodiment of the present disclosure;

FIG. 7 is a block diagram illustrating an example of an imaging systemconfigured to be capable of multiple-exposure images;

FIG. 8 is a diagram for describing an example of forming amultiple-exposure image;

FIG. 9 is a diagram illustrating together an exemplary multiple-exposureimage acquired by the imaging system illustrated in FIG. 7, and multipleimages, extracted in time-series from the multiple-exposure image, eachincluding one image of a moving body;

FIG. 10 is a diagram illustrating an example of a multiple-exposureimage, where an image of an identifier indicating temporal change of theposition of the moving body has been superimposed;

FIG. 11 is a diagram illustrating another example of a multiple-exposureimage, where an image of an identifier indicating temporal change of theposition of the moving body has been superimposed;

FIG. 12 is a diagram for describing another example of forming amultiple-exposure image;

FIG. 13 is a timing chart for describing exemplary operations of a resetvoltage source in a reset period;

FIG. 14 is a timing chart for describing exemplary operations of thereset voltage source in a reset period; and

FIG. 15 is a schematic diagram illustrating a modification of an imagingdevice.

DETAILED DESCRIPTION

The following is an overview of an aspect of the present disclosure.

Item 1

An imaging device includes: unit pixel cells each including a firstelectrode, a second electrode facing the first electrode, aphotoelectric conversion layer between the first electrode and secondelectrode, a charge accumulation region electrically connected to thefirst electrode, and a signal detection circuit electrically connectedto the charge accumulation region; and a voltage supply circuitelectrically connected to the second electrode, the voltage supplycircuit supplying a first voltage to the second electrode in an exposureperiod that is a period for accumulating charges generated byphotoelectric conversion in the charge accumulation region, the voltagesupply circuit supplying a second voltage that is different from thefirst voltage to the second electrode in a non-exposure period. Thestart and end of the exposure period is common to the unit pixel cells.

Item 2

The imaging device according to Item 1, wherein the unit pixel cellseach include a reset transistor that is electrically connected to thecharge accumulation region, the reset transistor switching betweensupply and cutoff of reset voltage for initializing the chargeaccumulation region, and a potential difference between the firstelectrode and the second electrode when the reset voltage is supplied isgreater than a potential difference between the first electrode and thesecond electrode after the reset voltage is cut off.

Item 3

The imaging device according to Item 2, wherein the reset transistor isan n-channel field-effect transistor, and the reset voltage is greaterthan the second voltage.

Item 4

The imaging device according to Item 2, wherein the reset transistor isa p-channel field-effect transistor, and the reset voltage is smallerthan the second voltage.

Item 5

The imaging device according to any one of Items 1 through 4, whereinthe unit pixel cells are two-dimensionally arrayed in rows and columns,and signals detected by the signal detection circuits of the unit pixelcells are read out at different timings for each of the rows.

Item 6

The imaging device according to any one of Items 1 through 5, whereinthe exposure period is one of a plurality of exposure periods, and theplurality of the exposure periods are included in one frame period.

Item 7

The imaging device according to Item 6, wherein the voltage supplycircuit supplies the first voltage to the second electrode at mutuallydifferent magnitudes among the plurality of exposure periods.

Item 8

The imaging device according to either Item 6 or 7 further includes animage forming circuit that acquires a plurality of sets of image databased on each output from the signal detection circuit at the pluralityof exposure periods, and forms a multiple-exposure image bysuperimposing the plurality of sets of image data.

Item 9

The imaging device according to either Item 6 or 7, further includes animage forming circuit that acquires a signal from the signal detectioncircuit, the signal corresponding to signal charges accumulated in thecharge accumulation region in the one frame period, the image formingcircuit forming a multiple-exposure image based on the signal.

Item 10

The imaging device according to any one of Items 1 through 9, whereinthe photoelectric conversion layer has a photocurrent characteristicbetween a bias voltage applied to the photoelectric conversion layer anda current density of a current flowing through the photoelectricconversion layer, the photocurrent characteristic including a firstvoltage range where an absolute value of the current density increasesas the bias voltage increases in a reverse direction, a second voltagerange where the current density increases as the bias voltage increasesin a forward direction, and a third voltage range where an absolutevalue of a rate of change of the current density relative to the biasvoltage is less than in the first voltage range and the second voltagerange, the third voltage range being between the first voltage range andthe second voltage range, and the voltage supply circuit supplies thesecond voltage to the second electrode in the non-exposure period suchthat the bias voltage applied to the photoelectric conversion layerfalls within the third voltage range.

Item 11

An imaging device includes:

a plurality of unit pixel cells each including a first electrode, acharge accumulation region electrically connected to the firstelectrode, and a signal detection circuit electrically connected to thecharge accumulation region;

a second electrode facing the first electrode;

a photoelectric conversion layer disposed between the first electrodeand the second electrode; and

a voltage supply circuit that is connected with the second electrode,and that supplies mutually different voltages to the second electrodebetween an exposure period and a non-exposure period, wherein

the photoelectric conversion layer has photocurrent characteristics inwhich change in rate of output current density as to bias voltagemutually differs in a first voltage range where an absolute value ofoutput current density increases along with increase in reverse biasvoltage, a second voltage range where an absolute value of outputcurrent density increases along with increase in forward bias voltage,and a third voltage range between the first voltage range and the secondvoltage range,

a rate of change in the third voltage range is smaller than the rate ofchange in the first voltage range and the second voltage range,

the start and end of the exposure period is held in common among theplurality of unit pixel cells, and

the voltage supply circuit supplies a voltage to the second electrode inthe non-exposure period, so as to impart a potential difference betweenthe second electrode and the signal detection circuit that falls withinthe third voltage range.

According to the configuration in Item 11, a global shutter can berealized without providing a separate transfer transistor and so forthwithin the unit pixel cell.

Item 12

The imaging device according to Item 11, wherein a plurality of theexposure periods are included in one frame period.

Item 13

The imaging device according to Item 12, wherein the voltage supplycircuit supplies voltage to the second electrode at mutually differentmagnitudes among the plurality of exposure periods.

According to the configuration in Item 13, imaging where sensitivity ischanged in each of the plurality of exposure periods can be performed.

Item 14

The imaging device according to either Item 12 or 13, further includingan image forming circuit that acquires a plurality of sets of image databased on each output from the signal detection circuits at the pluralityof exposure periods, and forms a multiple-exposure image bysuperimposing the plurality of sets of image data.

According to the configuration in Item 14, the path of an object movingover the period of one frame can be known from the multiple-exposureimage.

Item 15

The imaging device according to either Item 12 or 13, further includingan image forming circuit that acquires signals from the signal detectioncircuit, corresponding to signal charges accumulated in the chargeaccumulation region in the one frame period, and forms amultiple-exposure image based on the signals.

According to the configuration in Item 15, the path of an object movingover the period of one frame can be known from the multiple-exposureimage.

Item 16

The imaging device according to any one of Items 11 through 15, wherein

each of the unit pixel cells includes a reset transistor that iselectrically connected to the charge accumulation region, and thatswitches supply and cutoff of reset voltage to charge accumulationregion,

the reset transistor is an n-channel field-effect transistor, and

the reset voltage is greater than voltage that the voltage supplycircuit applies to the second electrode in a non-exposure period.

According to the configuration in Item 16, parasitic sensitivity can besuppressed more effectively.

Item 17

The imaging device according to any one of Items 11 through 15, wherein

each of the unit pixel cells includes a reset transistor that iselectrically connected to the charge accumulation region, and thatswitches supply and cutoff of reset voltage to charge accumulationregion,

the reset transistor is a p-channel field-effect transistor, and

the reset voltage is smaller than voltage that the voltage supplycircuit applies to the second electrode in a second period.

According to the configuration in Item 17, parasitic sensitivity can besuppressed more effectively.

Item 18

The imaging device according to either Item 16 or 17, wherein theabsolute value of difference between the reset voltage and the voltagethat the voltage supply current applies to the second electrode in anon-exposure period is smaller than the breakdown voltage of thephotoelectric conversion layer.

According to the configuration in Item 18, damage to the photoelectricconversion layer due to excessive application of voltage can be avoided.

Item 19

The imaging device according to either Item 16 or 17, wherein theabsolute value of difference between the reset voltage and the voltagethat the voltage supply current applies to the second electrode in anon-exposure period is smaller than the input voltage to the signaldetection circuit.

According to the configuration in Item 19, damage to the photoelectricconversion layer due to excessive application of voltage can be avoided.

Embodiments of the present disclosure will be described below withreference to the drawings. Note that the embodiments described below areeach general or specific examples. Values, shapes, materials,components, placements and connected states of components, steps, andthe order of steps, and so forth illustrated in the followingembodiments, are only exemplary, and do not restrict the presentdisclosure. Various aspects described in the present specification canbe combined as long as there is no contradiction. Components in thefollowing embodiments which are not included in an independent Claimindicating the highest concept are described as being optionalcomponents. Components having substantially the same functions may bedenoted by common reference symbols in the following description, anddescription thereof omitted.

Embodiment of Imaging Device

FIG. 1 illustrates an exemplary circuit configuration of an imagingdevice according to an embodiment of the present disclosure. An imagingdevice 100 illustrated in FIG. 1 has a pixel array PA that includesmultiple unit pixel cells 10 arrayed two-dimensionally. FIG. 1schematically illustrates an example where unit pixel cells 10 arearrayed in a matrix of two rows by two columns. It is needless to saythat the number and layout of the unit pixel cells 10 in the imagingdevice 100 are not restricted to the example illustrated in FIG. 1.

A unit pixel cell 10 has a photoelectric conversion unit 13 and a signaldetection circuit 14. The photoelectric conversion unit 13 has aphotoelectric conversion layer interposed between two mutually facingelectrodes, and generates signals upon receiving incident light. Theentire photoelectric conversion unit 13 does not need to be anindependent element for each unit pixel cell 10, and part of aphotoelectric conversion unit 13, for example, may span multiple unitpixel cells 10. The signal detection circuit 14 is a circuit thatdetects signals generated by the photoelectric conversion unit 13. Inthis example, the signal detection circuit 14 includes a signaldetection transistor 24 and an address transistor 26. The signaldetection transistor 24 and address transistor 26 typically arefield-effect transistors (FET). The signal detection transistor 24 andaddress transistor 26 here are exemplified as N-channel metal-oxidesemiconductor (MOS) transistors.

The control terminal (gate here) of the signal detection transistor 24has an electrical connection with the photoelectric conversion unit 13,as schematically illustrated in FIG. 1. Signal charges (holes orelectrons) generated by the photoelectric conversion unit 13 areaccumulated in a charge accumulation node (also referred to as “floatingdiffusion node”) 41 between the signal detection transistor 24 and thephotoelectric conversion unit 13. Details of the photoelectricconversion unit 13 will be described later.

The photoelectric conversion unit 13 of the unit pixel cell 10 furtherhas a connection with a sensitivity control line 42. The sensitivitycontrol line 42 is connected to a sensitivity control voltage supplycircuit 32 (hereinafter referred to simply as “voltage supply circuit32”) in the configuration exemplified in FIG. 1. This voltage supplycircuit 32 is a circuit configured to be capable of supplying at leasttwo types of voltage. The voltage supply circuit 32 suppliespredetermined voltage to the photoelectric conversion unit 13 via thesensitivity control line 42 when the imaging device 100 is operating.The voltage supply circuit 32 is not restricted to a particular powersource circuit, and may be a circuit that generates a predeterminedvoltage, or may be a circuit that converts voltage supplied from anotherpower source into predetermined voltage. Starting and ending ofaccumulation of signal charges from the photoelectric conversion unit 13to the charge accumulation node 41 is controlled by the voltage suppliedfrom the voltage supply circuit 32 to the photoelectric conversion unit13 being switched among multiple voltages that are different from eachother, which will be described later in detail. In other words,electronic shutter operations are executed in the embodiment accordingto the present disclosure by switching voltage supplied from the voltagesupply circuit 32 to the photoelectric conversion unit 13. An example ofoperations of the imaging device 100 will be described later.

Each unit pixel cell 10 has a connection with a power source line 40that supplies power source voltage VDD. The input terminal (typicallythe drain) of the signal detection transistor 24 is connected to thepower source line 40, as illustrated in FIG. 1. The signal detectiontransistor 24 amplifies and outputs signals generated by thephotoelectric conversion unit 13 due to the power source line 40functioning as a source-follower power source.

The output terminal (source here) of the signal detection transistor 24is connected to the input terminal (drain here) of the addresstransistor 26. The output terminal (source here) of the addresstransistor 26 is connected to one of multiple vertical signal lines 47arranged in each pixel array PA row. The control terminal (gate here) ofthe address transistor 26 is connected to an address control line 46,and the output of the signal detection transistor 24 can be selectivelyread out to the corresponding vertical signal line 47 by controlling thepotential of the address control line 46.

In the example illustrated in FIG. 1, the address control line 46 isconnected to a vertical scan circuit (also referred to as “row scancircuit”) 36. The vertical scan circuit 36 selects multiple unit pixelcells 10 arranged in each row by applying a predetermined voltage to theaddress control line 46. This executes readout of signals in theselected unit pixel cells 10, and later-described resetting of pixelelectrodes.

The vertical signal line 47 is a primary signal line transmitting pixelsignals from the pixel array PA to peripheral circuits. A column signalprocessing circuit (also referred to as “row signal accumulationcircuit”) 37 is connected to the vertical signal line 47. The columnsignal processing circuit 37 performs noise suppression signalprocessing, of which correlated double sampling is representative,analog-to-digital conversion (AD conversion), and so forth. A columnsignal processing circuit 37 is provided corresponding to each column ofunit pixel cells 10 in the pixel array PA, as illustrated in FIG. 1.Connected to these column signal processing circuits 37 are a horizontalsignal read circuit (also referred to as “column scan circuit”) 38. Thehorizontal signal read circuit 38 sequentially reads out signals fromthe column signal processing circuits 37 to a horizontal common signalline 49.

In the configuration exemplified in FIG. 1, the unit pixel cell 10 has areset transistor 28. The reset transistor 28 may be a field-effecttransistor, in the same way as the signal detection transistor 24 andaddress transistor 26, for example. An example will be described belowwhere an N-channel MOS is applied as the reset transistor 28, unlessspecifically stated otherwise. As illustrated in FIG. 1, the resettransistor 28 is connected between a reset voltage line 44 that suppliesreset voltage Vr and a charge accumulation node 41. The control terminal(gate here) of the reset transistor 28 is connected to a reset controlline 48, and the potential of the charge accumulation node 41 can bereset to the reset voltage Vr by controlling the potential of the resetcontrol line 48. The reset control line 48 is connected to the verticalscan circuit 36 is this example. Accordingly, multiple unit pixel cells10 arrayed in each row can be reset in increments of rows by thevertical scan circuit 36 applying predetermined voltage to the resetcontrol line 48.

In this example, the reset voltage line 44 that supplies reset voltageVr to the reset transistor 28 is connected to the reset voltage supplycircuit 34. It is sufficient that the configuration of the reset voltagesource 34 enables a predetermined reset voltage Vr to be supplied to thereset voltage line 44 when the imaging device 100 is operating, and isnot restricted to any particular power source circuit, the same as withthe voltage supply circuit 32 described above. The voltage supplycircuit 32 and reset voltage source 34 may each be part of a singlevoltage supply circuit, or may be independent and separate voltagesupply circuits. Note that one or both of the voltage supply circuit 32and reset voltage source 34 may be part of the vertical scan circuit 36.Alternatively, sensitivity control voltage from the voltage supplycircuit 32 and/or reset voltage Vr from the reset voltage source 34 maybe supplied to each unit pixel cell 10 via the vertical scan circuit 36.

Power source voltage VDD of the signal detection circuit 14 may be usedas the reset voltage Vr. In this case, a voltage supply circuit thatsupplies power source voltage to each of the unit pixel cells 10(omitted from illustration in FIG. 1) and the reset voltage source 34may be commonalized. The power source line 40 and reset voltage line 44can also be commonalized, so the wiring of the pixel array PA can besimplified. Note however, that using mutually different voltages for thereset voltage Vr and for the power source voltage VDD of the signaldetection circuit 14 enables more flexible control of the imaging device100.

Device Structure of Unit Pixel Cell

FIG. 2 schematically illustrates an exemplary device structure of theunit pixel cell 10. The above-described signal detection transistor 24,address transistor 26, and reset transistor 28, are formed on asemiconductor substrate 20 in the configuration exemplified in FIG. 2.The semiconductor substrate 20 is not restricted to a substrate of whichthe entirety is a semiconductor. The semiconductor substrate 20 may bean insulating substrate, where a semiconductor layer has been formed onthe surface of a side where a photosensitive region is formed, or thelike. An example of using a P-type silicon (Si) substrate as thesemiconductor substrate 20 will be described here.

The semiconductor substrate 20 includes impurity regions (N-type regionhere) 26 s, 24 s, 24 d, 28 d, and 28 s, and element separation region 20t for electric separation among unit pixel cells 10. The elementseparation region 20 t is also provided between impurity region 24 d andimpurity region 28 d as well. The element separation region 20 t isformed by injecting acceptor ions under predetermined injectionconditions, for example.

The impurity regions (N-type region here) 26 s, 24 s, 24 d, 28 d, and 28s, typically are diffusion layers formed within the semiconductorsubstrate 20. The signal detection transistor 24 includes the impurityregions 24 s and 24 d, and gate electrode 24 g (typically a polysiliconelectrode), as schematically illustrated in FIG. 2. The impurity region24 s functions as a source region, for example, of the signal detectiontransistor 24. The impurity region 24 d functions as a drain region, forexample, of the signal detection transistor 24. A channel region of thesignal detection transistor 24 is formed between the impurity regions 24s and 24 d.

In the same way, the address transistor 26 includes the impurity regions26 s and 24 s, and a gate electrode 26 g (typically a polysiliconelectrode) connected to the address control line 46 (see FIG. 1). Inthis example, the signal detection transistor 24 and address transistor26 are electrically connected to each other by sharing the impurityregion 24 s. The impurity region 26 s functions as a source region, forexample, of the address transistor 26. The impurity region 26 s has aconnection with the vertical signal line 47 (see FIG. 1) that is omittedfrom FIG. 2.

The reset transistor 28 has impurity regions 28 d and 28 s, and a gateelectrode 28 g (typically a polysilicon electrode) connected to thereset control line 48 (see FIG. 1). The impurity region 28 s functionsas a source region, for example, of the reset transistor 28. Theimpurity region 28 s has a connection with the reset voltage line 44(see FIG. 1) that is omitted from FIG. 2.

An inter-layer insulation layer 50 (typically a silicon dioxide layer)is disposed on the semiconductor substrate 20, covering the signaldetection transistor 24, address transistor 26, and reset transistor 28.A wiring layer 56 may be disposed in the inter-layer insulation layer50. The wiring layer 56 typically is formed of metal such as copper orthe like, and can include wiring such as the above-described verticalsignal line 47 and so forth as a part thereof, for example. The numberof layers of the insulating layer in the inter-layer insulation layer50, and the number of layers of the wiring layer 56 disposed in theinter-layer insulation layer 50 may be optionally set, and are notrestricted to the example illustrated in FIG. 2.

The above-described photoelectric conversion unit 13 is disposed on theinter-layer insulation layer 50. In other words, the multiple unit pixelcells 10 making up the pixel array PA (see FIG. 1) are formed on thesemiconductor substrate 20 in the embodiment according to the presentdisclosure. The unit pixel cells 10 arrayed two-dimensionally on thesemiconductor substrate 20 form a photosensitive region (pixel region).The distance between the centers of two adjacent unit pixel cells 10(pixel pitch) may be around 2 μm, for example.

The photoelectric conversion unit 13 includes a pixel electrode 11, anopposing electrode 12, and the photoelectric conversion layer 15interposed therebetween. The opposing electrode 12 and photoelectricconversion layer 15 are formed spanning multiple unit pixel cells 10 inthis example. On the other hand, the pixel electrode 11 is formed foreach unit pixel cell 10, and is electrically separated from pixelelectrodes 11 of other unit pixel cells 10 by being spatially separatedfrom pixel electrodes 11 of other unit pixel cells 10.

The opposing electrode 12 is typically a transparent electrode formed ofa transparent electroconductive material. The opposing electrode 12 isdisposed at the side where light enters the photoelectric conversionlayer 15. Accordingly, light that has passed through the opposingelectrode 12 enters the photoelectric conversion layer 15. Lightdetected by the imaging device 100 is not restricted to the wavelengthrange of visible light (e.g., 380 nm or more, and 780 nm or less). Theterm “transparent” as used in the present specification means that atleast part of light of a wavelength range to be detected is transmitted,and transmitting the entire wavelength range of visible light is notessential. For the sake of convenience, the electromagnetic waves ingeneral, including infrared rays and ultraviolet rays, will be expressedas “light”. For example, transparent conducting oxides (TCO) such asindium tin oxide (ITO), indium zinc oxide (IZO), aluminum-doped zincoxide (AZO), fluorine doped tin oxide (FTO), tin dioxide (SnO₂),titanium dioxide (TiO₂), zinc dioxide (ZnO₂), and so forth, can be usedas the opposing electrode 12.

The photoelectric conversion layer 15 receives incident light, andgenerates hole-electron pairs. The photoelectric conversion layer 15typically is formed of an organic material. Specific examples ofmaterials configuring the photoelectric conversion layer 15 will bedescribed later.

As described referring to FIG. 1, the opposing electrode 12 has aconnection with the sensitivity control line 42 connected to the voltagesupply circuit 32. The opposing electrode 12 here is formed spanningmultiple unit pixel cells 10. Accordingly, sensitivity control voltageof a predetermined magnitude can be applied en bloc from the voltagesupply circuit 32 to the unit pixel cells 10 via the sensitivity controlline 42. Note that the opposing electrode 12 may be separated for eachunit pixel cell 10, as long as sensitivity control voltage of apredetermined magnitude can be applied from the voltage supply circuit32. In the same way, the photoelectric conversion layer 15 may beseparated for each unit pixel cell 10.

The voltage supply circuit 32 supplies mutually different voltages tothe opposing electrode 12, depending on whether during an exposureperiod or a non-exposure period, which will be described later indetail. The term “exposure period” in the present specification means aperiod for accumulating one of positive and negative charges (signalcharges) generated by photoelectric conversion in the chargeaccumulation region, and may be referred to as “charge accumulationperiod”. A period during operations of the imaging device other than anexposure period is referred to as “non-exposure period” in the presentspecification. Note that “non-exposure period” is not restricted to aperiod when input of light to the photoelectric conversion unit 13 isshielded, and may include a period when the photoelectric conversionunit 13 is being irradiated by light. “Non-exposure period” alsoincludes a period when signal charges are unintentionally accumulated inthe charge accumulation region due to occurrence of parasiticsensitivity.

Controlling the potential of the opposing electrode 12 relative to thepotential of the pixel electrode 11 enables one of holes and electrons,of the hole-electron pairs generated in the photoelectric conversionlayer 15 by photoelectric conversion, to be collected by the pixelelectrode 11. For example, in a case of using holes as signal charges,holes can be selectively collected by the pixel electrode 11 by settingthe potential of the opposing electrode 12 higher than the potential ofthe pixel electrode 11. A case of using holes as signal charges will beexemplified below. Of course, electrons can be used as signals chargesas well.

Applying an appropriate bias voltage between the opposing electrode 12and pixel electrode 11 causes the pixel electrode 11 facing the opposingelectrode 12 to collect one of positive and negative charges generatedby photoelectric conversion at the photoelectric conversion layer 15.The pixel electrode 11 is formed of a metal such as aluminum, copper, orthe like, a metal nitride, polysilicon that has been impartedelectroconductivity by doping with an impurity, or the like.

The pixel electrode 11 may be a light-shielding electrode. For example,forming a tantalum nitride (TaN) electrode 100 nm thick as the pixelelectrode 11 realizes sufficient light shielding characteristics.Forming the pixel electrode 11 as a light-shielding electrode enableslight that has passed through the photoelectric conversion layer 15 tobe suppressed from entering the channel region or impurity region oftransistors (at least one of the signal detection transistor 24, addresstransistor 26, and reset transistor 28 in this example) formed on thesemiconductor substrate 20. The above-described wiring layer 56 may beused to form a light-shielding layer in the inter-layer insulation layer50. Suppressing light from entering the channel region of transistorsformed on the semiconductor substrate 20 enables shifting of transistorcharacteristics (e.g., change in threshold voltage) and so forth to besuppressed. Suppressing light from entering the impurity region formedon the semiconductor substrate 20 enables unintended noise due tophotoelectric conversion in the impurity region from being included.Thus, suppressing light from entering the semiconductor substrate 20contributes to improved reliability of the imaging device 100.

The pixel electrode 11 is connected to the gate electrode 24 g of thesignal detection transistor 24 via a plug 52, wiring 53, and a contactplug 54, as schematically illustrated in FIG. 2. In other words, thegate of the signal detection transistor 24 has electric connection withthe pixel electrode 11. The plug 52 and wiring 53 are formed of metalsuch as copper, for example. The plug 52, wiring 53, and contact plug 54make up at least part of the charge accumulation node 41 (see FIG. 1)between the signal detection transistor 24 and the photoelectricconversion unit 13. The wiring 53 may be part of the wiring layer 56.The pixel electrode 11 is also connected to the impurity region 28 d viathe plug 52, wiring 53, and a contact plug 55. In the configurationillustrated in FIG. 2, the gate electrode 24 g of the signal detectiontransistor 24, the plug 52, wiring 53, contact plugs 54 and 55, and theimpurity region 28 d that is one of the source region and drain regionof the reset transistor 28, function as the charge accumulation regionaccumulating signal charges collected by the pixel electrode 11.

By collecting signal charges to the pixel electrode 11, voltagecorresponding to the quantity of signal charges accumulated in thecharge accumulation region is applied to the gate of the signaldetection transistor 24. The signal detection transistor 24 amplifiesthis voltage. The voltage amplified by the signal detection transistor24 is selectively read out via the address transistor 26 as signalvoltage.

Findings of Present Inventors and Typical Example of Configuration ofPhotoelectric Conversion Layer

As described above, irradiating the photoelectric conversion layer 15 bylight and applying bias voltage between the pixel electrode 11 andopposing electrode 12 enables one of positive and negative chargesgenerated by photoelectric conversion to be collected by the pixelelectrode 11, and the collected charges to be accumulated in the chargeaccumulation region. The present inventors have found that movement ofsignal charges already accumulated in the charge accumulation region tothe opposing electrode 12 via the photoelectric conversion layer 15 canbe suppressed by using a photoelectric conversion layer 15, havingphotocurrent characteristics such as described below, in thephotoelectric conversion unit 13 and reducing the potential differencebetween the pixel electrode 11 and opposing electrode 12 to a certainlevel. The present inventors have further found that furtheraccumulation of signal charges in the charge accumulation region can besuppressed after reducing the potential difference. That is to say, ithas been found that global shutter functions can be realized bycontrolling the magnitude of bias voltage applied to the photoelectricconversion layer 15, without separately providing elements such as atransfer transistor to each of the multiple pixels. A typical example ofoperations at the imaging device 100 will be described later.

An example of the configuration of the photoelectric conversion layer15, and photocurrent characteristics of the photoelectric conversionlayer 15, will be described below. The photoelectric conversion layer 15typically contains a semiconductor material. An organic semiconductormaterial is used here as the semiconductor material. The photoelectricconversion layer 15 includes tin naphthalocyanine expressed by thegeneral formula (1) below (hereinafter may be referred to simply as “tinnaphthalocyanine”).

In the general formula (1), R¹ through R²⁴ independently represent ahydrogen atom or substituent group. Substituent groups are notrestricted to particular substituent groups. A substituent group may bea deuterium atom, halogen atom, alkylic group (including cycloalkylgroup, bicycloalkyl group, tricycloalkyl group), alkenyl group(including cycloalkenyl group and bicycloalkenyl group), alkynyl group,aryl group, heterocyclic group (may also be called hetero ring group),cyano group, hydroxy group, nitro group, carboxy group, alkoxy group,aryloxy group, silyloxy group, heterocyclic oxy group, acyloxy group,carbamoyloxy group, alkoxycarbonyloxy group, aryloxycarbonyloxy group,amino group (including anilino group), ammonio group, acylamino group,aminocarbonyl amino group, alkoxycarbonylamino group, aryloxycarbonylamino group, sulfamoyl amino group, alkylsulfonyl amino group,arylsulfonylamino group, mercapto group, alkylthio group, arylthiogroup, heterocyclic thio group, sulfamoyl group, sulfo group,alkylsulfinyl group, arylsulfinyl group, alkylsulfonyl group,arylsulfonyl group, acyl group, aryloxycarbonyl group, alkoxycarbonylgroup, carbamoyl group, arylazo group, heterocyclic azo group, imidegroup, phosphino group, phosphinyl group, phosphinyloxy group, hophinylamino group, phosphono group, silyl group, hydrazino group, ureidogroup, boronic acid group (—B(OH)₂), phosphato group (—OPO(OH)₂),sulfato group (—OSO₃H), or other known substituent groups.

Commercially-available products can be used for the tin naphthalocyaninein the above-described general formula (1). Alternatively, the tinnaphthalocyanine in the above-described general formula (1) may besynthesized using a naphthalene derivative shown in the followinggeneral formula (2) as the starting material, as described in JapaneseUnexamined Patent Application Publication No. 2010-232410. R²⁵ throughR³⁰ in general formula (2) may be substituents the same as R¹ throughR²⁴ in general formula (1).

From the perspective of ease of controlling the coagulation state ofmolecules, it is desirable that in the tin naphthalocyanine in theabove-described general formula (1), eight or more of the R¹ through R²⁴are hydrogen atoms or deuterium atoms, more desirable that 16 or more ofthe R¹ through R²⁴ are hydrogen atoms or deuterium atoms, and even moredesirable that all of the R¹ through R²⁴ are hydrogen atoms or deuteriumatoms. Further, the tin naphthalocyanine shown in the following generalformula (3) is advantageous from the perspective of ease of synthesis.

The tin naphthalocyanine in the above-described general formula (1) hasabsorption in a wavelength band generally 200 nm or more and 1100 nm orless. For example, the tin naphthalocyanine in the general formula (3)has an absorption peak at a position around a wavelength of 870 nm, asillustrated in FIG. 3. FIG. 3 is an example of an absorption spectrum ina photoelectric conversion layer containing the tin naphthalocyanineshown in the general formula (3). Note that measurement of theabsorption spectrum as performed using a sample where a photoelectricconversion layer (30 nm thick) was deposited on a quartz substrate.

It can be seen from FIG. 3 that a photoelectric conversion layer formedof material including tin naphthalocyanine has absorption in anear-infrared region. That is to say, selecting a material including tinnaphthalocyanine as the material for configuring the photoelectricconversion layer 15 enables a light sensor that can detect near-infraredrays to be realized, for example.

FIG. 4 schematically illustrates an example of the configuration of thephotoelectric conversion layer 15. In the configuration exemplarilyillustrated in FIG. 4, the photoelectric conversion layer 15 includes ahole blocking layer 15 h, a photoelectric conversion structure 15Aformed using an organic semiconductor material including the tinnaphthalocyanine in the above-described general formula (1), and anelectron blocking layer 15 e. The hole blocking layer 15 h is disposedbetween the photoelectric conversion structure 15A and opposingelectrode 12, and the electron blocking layer 15 e is disposed betweenthe photoelectric conversion structure 15A and pixel electrode 11.

The photoelectric conversion structure 15A illustrated in FIG. 4includes at least one of a p-type semiconductor and n-typesemiconductor. In the configuration exemplarily illustrated in FIG. 4,the photoelectric conversion structure 15A includes a p-typesemiconductor layer 150 p, an n-type semiconductor layer 150 n, and amixed layer 150 m interposed between the p-type semiconductor layer 150p and n-type semiconductor layer 150 n. The p-type semiconductor layer150 p is disposed between the electron blocking layer 15 e and the mixedlayer 150 m, and has photoelectric conversion and/or hole transportingfunctions. The n-type semiconductor layer 150 n is disposed been thehole blocking layer 15 h and the mixed layer 150 m, and hasphotoelectric conversion and/or electron transporting functions. Themixed layer 150 m may contain at least one of a p-type semiconductor andan n-type semiconductor, which will be described later.

The p-type semiconductor layer 150 p and the n-type semiconductor layer150 n respectively include an organic p-type semiconductor and anorganic n-type semiconductor. That is to say, the photoelectricconversion structure 15A includes an organic photoelectric conversionmaterial including the tin naphthalocyanine in the above-describedgeneral formula (1), and at least one of an organic p-type semiconductorand an organic n-type semiconductor.

The organic p-type semiconductor (compound) is a donor organicsemiconductor (compound) and is an organic compound that is primarilyrepresented by hole-transporting organic compounds and has a nature ofreadily donating electrons. More specifically, the organic p-typesemiconductor (compound) is an organic compound that has the smallerionization potential of two organic materials when the two organicmaterials are used in contact. Accordingly, any organic compound can beused as the donor organic compound as long as it is an electron-donatingorganic compound. Examples include a triarylamine compound, benzidinecompound, pyrazoline compound, styrylamine compound, hydrazone compound,triphenylmethane compound, carbazole compound, polysilane compound,thiophene compound, phthalocyanine compound, cyanine compound,merocyanine compound, oxonol compound, polyamine compound, indolecompound, pyrrole compound, pyrazole compound, polyarylene compound,condensed aromatic carbocyclic compound (naphthalene derivative,anthracene derivative, phenanthrene derivative, tetracene derivative,pyrene derivative, perylene derivative, fluoranthene derivative),metallic complex having a nitrogen-containing heterocyclic compound as aligand, and so forth. Note that donor organic semiconductors are notrestricted to those, and any organic compound can be used as the donororganic semiconductors as long as it has an ionization potential smallerthan an organic compound used as an n-type (acceptor) compound. Theabove-described tin naphthalocyanine is an example of an organic p-typesemiconductor material.

The organic n-type semiconductor (compound) is an acceptor organicsemiconductor (compound) and is an organic compound that is primarilyrepresented by electron-transporting organic compounds and has a natureof readily accepting electrons. More specifically, the organic n-typesemiconductor (compound) is an organic compound that has the greaterelectron affinity of two organic materials when the two organicmaterials are used in contact. Accordingly, any organic compound can beused as the acceptor organic compound as long as it is anelectron-accepting organic compound. Examples include fullerene,fullerene derivative, condensed aromatic carbocyclic compound(naphthalene derivative, anthracene derivative, phenanthrene derivative,tetracene derivative, pyrene derivative, perylene derivative,fluoranthene derivative), five- to seven-membered heterocyclic compoundsincluding nitrogen atoms, oxygen atoms, or sulfur atoms (e.g., pyridine,pyrazine, pyrimidine, pyridazine, triazine, quinoline, quinoxaline,quinazoline, phthalazine, cinnoline, isoquinoline, pteridine, acridine,phenazine, phenanthroline, tetrazole, pyrazole, imidazole, thiazole,oxazole, indazole, benzimidazole, benzotriazole, benzoxazole,benzothiazole, carbazole, purine, triazolopyridazine,triazolopyrimidine, tetrazaindene, oxadiazole, imidazopyridine,piridine, pyrrolopyridine, thiadiazolopyridine, dibenzazepine,tribenzazepine, etc.), polyarylene compound, fluorene compound,cyclopentadiene compound, silyl compound, metallic complex having anitrogen-containing heterocyclic compound as a ligand, and so forth.Note that acceptor organic semiconductors are not restricted to those,and any organic compound can be used as the acceptor organicsemiconductor as long as it has an electron affinity greater than anorganic compound used as a p-type (donor) compound.

The mixed layer 150 m may be a bulk heterojunction structure including ap-type semiconductor and an n-type semiconductor, for example. In a caseof forming the mixed layer 150 m as a layer having a bulk heterojunctionstructure, the tin naphthalocyanine in the above-described generalformula (1) may be used as the p-type semiconductor material. Fullereneand/or a fullerene derivative, for example, may be used as the n-typesemiconductor material. It is advantageous for the material making upthe p-type semiconductor layer 150 p to be the same as the p-typesemiconductor material included in the mixed layer 150 m. In the sameway, it is advantageous for the material making up the n-typesemiconductor layer 150 n to be the same as the n-type semiconductormaterial included in the mixed layer 150 m. A bulk heterojunctionstructure is disclosed in Japanese Patent No. 5553727. The contents ofthe disclosure in Japanese Patent No. 5553727 are hereby incorporated inthe present specification for reference.

Using an appropriate material in accordance with the wavelength bandregarding which detection is desired enables an imaging device havingsensitivity regarding the desired wavelength band to be realized. Thephotoelectric conversion layer 15 may include inorganic semiconductormaterial such as amorphous silicon and the like. The photoelectricconversion layer 15 may include a layer made up of organic material anda layer made up of inorganic material. An example of applying a bulkheterojunction structure, obtained by codeposition of tinnaphthalocyanine and C60, to the photoelectric conversion layer 15, willbe described below.

Photocurrent Characteristics in Photoelectric Conversion Layer

FIG. 5 illustrates typical photocurrent characteristics of thephotoelectric conversion layer 15. The graph with the thick solid linein FIG. 5 represents exemplary current-voltage characteristics (I-Vcharacteristics) of the photoelectric conversion layer 15. Note thatFIG. 5 also illustrates an example of I-V characteristics in a statewhere there is no irradiation by light, by the thick dotted line.

FIG. 5 illustrates change in current density between two principal facesof the photoelectric conversion layer 15 when bias voltage appliedtherebetween is changed. In the present specification, the forwarddirection and reverse direction in bias voltage is defined as follows.In a case where the photoelectric conversion layer 15 has a junctionstructure of a layered p-type semiconductor and a layered n-typesemiconductor, bias voltage where the potential of the p-typesemiconductor layer is higher than that of the n-type semiconductorlayer is defined as forward bias voltage. On the other hand, biasvoltage where the potential of the p-type semiconductor layer is lowerthan that of the n-type semiconductor layer is defined as reverse biasvoltage. Forward and reverse can be defined in a case of using organicsemiconductor material, in the same way as a case of using inorganicsemiconductor material. In a case where the photoelectric conversionlayer 15 has a bulk heterojunction structure, more p-type semiconductorthan n-type semiconductor appears on one surface of the two principalfaces of the bulk heterojunction structure, and more n-typesemiconductor than p-type semiconductor appears on the other surface, asschematically illustrated in FIG. 1 of Japanese Patent No. 5553727described above. Accordingly, bias voltage where the potential of theprincipal face where more p-type semiconductor than n-type semiconductorappears, is higher than that of the principal face where more n-typesemiconductor than p-type semiconductor appears, is defined as forwardbias voltage.

The photocurrent characteristics of the photoelectric conversion layer15 are schematically characterized by three voltage ranges, which arethe first through third voltage ranges illustrated in FIG. 5. The firstvoltage range is a reverse bias voltage range, and is a voltage rangewhere the absolute value of output current density increases along withincrease in reverse bias voltage. The first voltage range can be said tobe a voltage range where photocurrent increases along with increase inbias voltage applied between the principal faces of the photoelectricconversion layer 15. The second voltage range is a forward bias voltagerange, and is a voltage range where the absolute value of output currentdensity increases along with increase in forward bias voltage. That isto say, the second voltage range is a voltage range where forwardcurrent increases along with increase in bias voltage applied betweenthe principal faces of the photoelectric conversion layer 15. The thirdvoltage range is a voltage range between the first voltage range and thesecond voltage range.

The first through third voltage ranges are distinguished by theinclination of the photocurrent characteristic graph when a linearvertical axis and a linear horizontal axis are used. For reference, theaverage inclinations of the graph in the first voltage range and thesecond voltage range are respectively indicated by a dotted line L1 anddotted line L2. The rate of change of output current density relative toincrease of bias voltage differs among the first voltage range, secondvoltage range, and third voltage range, as exemplarily illustrated inFIG. 5. The third voltage range is defined as a voltage range where therate of change of output current density relative to bias voltage issmaller than the rate of change in the first voltage range and the rateof change in the second voltage range. Alternatively, the third voltagerange may be decided based on the position where the graph representingI-V characteristics rises (falls). The third voltage range typically isgreater than −1 V and smaller than +1 V. Changing the bias voltagewithin the third voltage range hardly changes the current densitybetween principal faces of the photoelectric conversion layer 15. Theabsolute value of current density in the third voltage range typicallyis 100 μmA/cm² or less.

Example of Operations of Imaging Device 100

FIG. 6 is a diagram for describing an example of operations of theimaging device according to the embodiment of the present disclosure.FIG. 6 illustrates the timing of the trailing edge (or leading edge) ofa synchronization signal, change over time of the magnitude of biasvoltage applied to the photoelectric conversion layer 15, and the timingof resetting and exposure in each row of the pixel array PA (see FIG.1). More specifically, the topmost graph in FIG. 6 indicates the timingof the trailing edge (or leading edge) of a vertical synchronizationsignal Vss. The second graph from the top indicates the timing of thetrailing edge (or leading edge) of a horizontal synchronization signalHss. Beneath these graphs are illustrated an example of change over timeof voltage Vb applied from the voltage supply circuit 32 to the opposingelectrode 12 via the sensitivity control line 42. Under the graph ofchange over time of voltage Vb is change over time of potential ϕ of theopposing electrode 12 with the potential of the pixel electrode 11 as areference. The both-sided arrow G3 at the potential ϕ graph indicatesthe above-described third voltage range. The chart further belowschematically illustrates the timing of resetting and exposure at eachrow in the pixel array PA.

Hereinafter, an example of operations at the imaging device 100 will bedescribed with reference to FIGS. 1, 2, and 6. For the sake of brevity,an example of operations in a case where the number of rows of pixelsincluded in the pixel array PA is the eight rows of row R0 through rowR7.

In the acquisition of an image, first, resetting of the chargeaccumulation region of each unit pixel cell 10 in the pixel array PA,and pixel signal readout after resetting, is performed. For example,resetting is started of the multiple pixels belonging to the R0 row,based on the vertical synchronization signal Vss (time t0). Note thatthe rectangles indicated by dots in FIG. 6 schematically representsignal readout periods. These readout periods may include a reset periodfor resetting the potential of the charge accumulation region of theunit pixel cell 10 therein.

In the resetting of pixels belonging to the R0 row, the addresstransistor 26 the gate of which is connected to the address control line46 is turned ON by control of the potential of the address control line46 for the row R0, and further, the reset transistor 28 the gate ofwhich is connected to the reset control line 48 is turned ON by controlof the potential of the reset control line 48 for the row R0.Accordingly, the charge accumulation node 41 and reset voltage line 44are connected to each other, and reset voltage Vr is supplied to thecharge accumulation region. That is to say, the potential of the gateelectrode 24 g of the signal detection transistor 24 and the pixelelectrode 11 of the photoelectric conversion unit 13 is reset to thereset voltage Vr. Thereafter, after resetting, pixel signals are readout from the unit pixel cells 10 in the row R0 via the vertical signalline 47. The pixel signals obtained at this time are pixel signalscorresponding to the magnitude of the reset voltage Vr. After readingout of the pixel signals, the reset transistor 28 and address transistor26 are turned off.

In this example, resetting of pixels belonging to the rows of row R0through row R7 in accordance with the horizontal synchronization signalHss is sequentially executed, as schematically illustrated in FIG. 6.Hereinafter, intervals between horizontal synchronization signal Hsspulses, i.e., a period from the time when one row is selected to thetime when the next row is selected, may be referred to as “one Hperiod”. The period from time t0 to time t1 is equivalent to one Hperiod in this example.

During the period from the time when image acquisition starts to thetime when resetting and readout of the pixel signal of all pixel arraysPA ends (time t0 through t9), a voltage V3 is applied from the voltagesupply circuit 32 to the opposing electrode 12 such that the potentialdifference between the pixel electrode 11 and opposing electrode 12falls within the above-described third voltage range. That is to say, inthe period from starting of image acquisition to starting of theexposure period (time t9), the photoelectric conversion layer 15 of thephotoelectric conversion unit 13 is in a state where bias voltage in thethird voltage range is applied.

In a state where bias voltage in the third voltage range is applied tothe photoelectric conversion layer 15, any movement of signal chargesfrom the photoelectric conversion layer 15 to the charge accumulationregion hardly occurs. The reason is estimated to be that in the statewhere bias voltage of the third voltage range is applied to thephotoelectric conversion layer 15, almost all of the positive andnegative changes generated by irradiation by light rapidly recouple, andvanish before being collected by the pixel electrode 11. Accordingly,even in a case where light enters the photoelectric conversion layer 15,accumulation of signals charges to the charge accumulation region hardlyoccurs at all in the state where the bias voltage of the third voltagerange is applied to the photoelectric conversion layer 15. Thus,occurrence of unintended sensitivity (may be referred to as “parasiticsensitivity” in the present specification) in a period other than anexposure period is suppressed. In this way, the fact that sensitivitycan be quickly dropped to 0 by setting the bias voltage to thephotoelectric conversion layer 15 to fall within the third voltage rangeis a finding that has been first discovered by the present inventors.

When focusing on a certain row in FIG. 6 (row R0 for example), theperiods of rectangles indicated by oblique lines and rectanglesindicated by dots represent non-exposure periods. Note that the voltageV3 for applying bias voltage within the third voltage range to thephotoelectric conversion layer 15 is not restricted to 0 V.

After resetting and reading out of pixel signals for all rows in thepixel array PA has ended, the exposure period is started based on thehorizontal synchronization signal Hss (time t9). The white rectangles inFIG. 6 schematically represent exposure periods at each row. Theexposure period is started by the voltage supply circuit 32 switchingthe voltage to be applied to the opposing electrode 12 to a voltage Vethat is different from the voltage V3. The voltage Ve typically is avoltage where the potential difference between the pixel electrode 11and opposing electrode 12 falls within the above-described first voltagerange (e.g., around 10 V). Due to the voltage Ve being applied to theopposing electrode 12, signal changes in the photoelectric conversionlayer 15 (holes in this example) are collected by the pixel electrode 11and accumulated in the charge accumulation region (may be referred to ascharge accumulation node 41).

The voltage supply circuit 32 switches the voltage applied to theopposing electrode 12 to the voltage V3 again, whereby the exposureperiod ends (time t13). Thus, in the embodiment according to the presentdisclosure, the exposure period and non-exposure period are switched byswitching the voltage applied to the opposing electrode 12 betweenvoltage V3 and voltage Ve. It can be seen from FIG. 6 that the start(time t9) and end (time t13) of the exposure period is held in commonamong all pixels included in the pixel array PA. That is to say, theoperations described here are an example of a global shutter applied tothe imaging device 100.

Next, readout of signal charges from the pixels belonging to each of therows in the pixel array PA is started, based on the horizontalsynchronization signal Hss. In this example, readout of signals chargesfrom pixels belonging to the rows of row R0 through R7 is sequentiallyperformed in increments of rows, from time t15. Hereinafter, a periodfrom the time when pixels belonging to a certain row are selected to thetime when pixels belonging to that row are selected again may bereferred to as “1 V period”. A period from time t0 to time 15 isequivalent to 1 V period in this example.

In the readout of signal charges from the pixels belonging to the row R0after the exposure period ends, the address transistor 26 of the row R0is turned on. Accordingly, pixel signals corresponding to the amount ofcharges accumulated in the charge accumulation region during theexposure period are output to the vertical signal line 47. Followingreadout of the pixel signals, the reset transistor 28 may be turned onto reset the pixels. After readout of the pixel signals, the addresstransistor 26 (and reset transistor 28) are turned off. After readout ofthe signal changes from the pixels belonging to each of the rows on thepixel array PA, the differences between signals from the signal chargesand signals read out during time t0 to t9 are obtained, thereby yieldingsingals from which static noise has been removed.

Since voltage V3 is applied to the opposing electrode 12 during thenon-exposure period, the photoelectric conversion layer 15 of thephotoelectric conversion unit 13 is in a state where bias voltage withinthe third voltage range is applied thereto. Accordingly, furtheraccumulation of signal charges to the charge accumulation region hardlyoccurs even if light enters the photoelectric conversion layer 15.Accordingly, occurrence of noise due to inclusion of unintended changesis suppressed.

An arrangement may be conceived where the exposure period is ended byapplying voltage, which has an inverted polarity of the above-describedvoltage Ve, to the opposing electrode 12, from the perspective ofsuppressing further accumulation of signal charges to the chargeaccumulation region. However, simply inverting the polarity of thevoltage applied to the opposing electrode 12 may cause movement ofalready-accumulated signal charges to the opposing electrode 12 via thephotoelectric conversion layer 15. Movement of signal charges to theopposing electrode 12 via the photoelectric conversion layer 15 will beobserved as black spots in the acquired image. That is to say, movementof signal charges from the charge accumulation region to the opposingelectrode 12 via the photoelectric conversion layer 15 can become thecause of negative parasitic sensitivity.

In this example, since the voltage applied to the opposing electrode 12is changed to voltage V3 again after the exposure period has ended, thephotoelectric conversion layer 15, after accumulation of signal chargesto the charge accumulation region, is in a state where the bias voltagein the third voltage range is applied. In the state where bias voltagein the third voltage range is applied, signal charges alreadyaccumulated in the charge accumulation region can be suppressed frommoving to the opposing electrode 12 via the photoelectric conversionlayer 15. In other words, signal changes accumulated during the exposureperiod can be held in the charge accumulation region by application ofthe bias voltage in the third voltage range to the photoelectricconversion layer 15. That is to say, occurrence of negative parasiticsensitivity due to loss of signal charges from the charge accumulationregion can be suppressed.

Thus, the starting and ending of the exposure period is controlled byvoltage Vb applied to the opposing electrode 12 in the embodiment of thepresent disclosure. That is to say, functions of a global shutter can berealized by the embodiment of the present disclosure without providingtransfer transistors and so forth within each unit pixel cell 10. Anelectronic shutter is executed in the embodiment of the presentdisclosure by controlling the voltage Vb without transferring signalcharges via a transfer transistor, so higher speed operations can berealized. Also, the transfer transistor and the like do not have to beprovided within each unit pixel cell 10, which is advantageous inminiaturization of pixels.

Application Example

In the example of operations described with reference to FIG. 6, oneexposure period is set in common for all pixels within 1 V period, andone image is acquired based on signal charges accumulated within thatexposure period. In such operations, the total amount of time needed toacquire pixel signals necessary for forming a final image, that is, oneframe worth of image can be said to be approximately equal to (1 Vperiod)+(number of rows in pixel array PA)×(readout time of signals),“×” meaning multiplication. The total amount of time needed to acquirepixel signals necessary for forming one frame worth of image will bereferred to as “one frame period” in the present specification. In theexample illustrated in FIG. 6, the readout period for signals is equallyset to one H period for each of the rows in the pixel array PA, so oneframe period can be said to be (1V+8×1 H).

In the example illustrated in FIG. 6, one exposure period is set incommon for all pixels in one frame period. However, multiple exposureperiods may be set in common for all pixels in one frame period. Inother words, multiple exposure may be performed, with one frame imagefinally being formed. The path of an object that has moved during oneframe period (hereinafter may be referred to as “moving body”) can berecorded during the recording of one frame in multiple exposure.Multiple exposure is useful in analysis of moving bodies and analysis ofhigh-speed phenomena. Hereinafter, an image formed based on pixelsingals obtained by executing multiple exposure will be referred to as a“multiple-exposure image”.

FIG. 7 schematically illustrates an example of an imaging systemconfigured to be able to form multiple-exposure images. The imagingsystem 100S exemplified in FIG. 7 schematically includes a camera unit80 and a display unit 90. The camera unit 80 and display unit 90 may betwo parts of a single device, or each may be independent and separatedevices. In the configuration exemplified in FIG. 7, the camera unit 80has an optical system 110, an imaging device 100, a system controller120, and an image formation circuit 130. The display unit 90 includes asignal processing circuit 150 and a display device 160.

The optical system 110 of the camera unit 80 includes a diaphragm, animage stabilization lens, zoom lens, focusing lens, and so forth. Thenumber of lenses that the optical system 110 has is decided asappropriate in accordance with the functions that are required. Thesystem controller 120 controls the various parts of the camera unit 80.The system controller 120 typically is a semiconductor integratedcircuit such as a central processing unit (CPU) or the like, and sendsout control singals to a lens driving circuit in the optical system 110,for example. The system controller 120 in this example also controls theoperations of the imaging device 100. For example, the system controller120 controls driving of the vertical scan circuit 36. Switching ofvoltage applied from the voltage supply circuit 32 to the sensitivitycontrol line 42 may be executed based on control by the systemcontroller 120. The system controller 120 may include one or more memorydevices. The image formation circuit 130 is configured to form amultiple-exposure image based on output of the imaging device 100. Theimage formation circuit 130 may be a digital signal processor (DSP),field-programmable gate array (FPGA), or the like, for example. Theimage formation circuit 130 may include memory. The operations of theimage formation circuit 130 may be controlled by the system controller120. An example of formation of a multiple-exposure image will bedescribed later.

The image formation circuit 130 has an output buffer 140 in theconfiguration exemplified in FIG. 7. The image formation circuit 130outputs data of a multiple-exposure image to the display unit 90 via theoutput buffer 140. The data output from the image formation circuit 130typically is RAW data, that is, 12 bit wide, for example. Data outputfrom the image formation circuit 130 may be data compressed confirmingto the H.264 standard, for example.

The signal processing circuit 150 of the display unit 90 receives outputfrom the image formation circuit 130. The output from the imageformation circuit 130 may be temporarily saved in an external recordingmedium configured to be detachably connected to the camera unit 80(e.g., flash memory). That is to say, output from the image formationcircuit 130 may be handed to the display unit 90 via the externalrecording medium.

The signal processing circuit 150 performs processing such as gammacorrection, color interpolation, spatial interpolation, auto whitevalance, and so forth. The signal processing circuit 150 typically is aDSP, an image signal processor (ISP), or the like. The display device160 of the display unit 90 is a liquid crystal display, organic EL(electroluminescence) display, or the like. The display device 160displays images based on output signals from the signal processingcircuit 150. The display unit 90 may be a personal computer, smartphone,or the like.

An example of forming a multiple-exposure image will be described belowwith reference to FIGS. 8 through 12. FIG. 8 is a diagram for describingan example of forming a multiple-exposure image. Exposure is executedmultiple times in the formation of one frame worth of amultiple-exposure image. First, resetting of pixels belonging to therows of row R0 through row R7 and readout of pixel signals in accordancewith the vertical synchronization signal Vss is sequentially executed inincrements of rows, as illustrated in FIG. 8 (time t00). The voltagesupply circuit 32 (see FIG. 1) applies voltage V3 such that thepotential difference between the pixel electrode 11 and opposingelectrode 12 falls within the above-described third voltage range.

Next, the voltage applied to the opposing electrode 12 is switched tovoltage Ve1, thereby starting the exposure period of all pixels in thepixel array PA in common. The voltage Ve1 is a voltage where thepotential difference between the pixel electrode 11 and opposingelectrode 12 falls within the above-described first voltage range, forexample. Applying the voltage Ve1 to the opposing electrode 12 causesone of positive and negative charges (signal charges) generated byphotoelectric conversion to be accumulated in the charge accumulationregion. The exposure period ends by the voltage supply circuit 32switching the voltage applied to the opposing electrode 12 to voltage V3again.

Next, readout of pixel signals of pixels belonging to the rows of row R0through row R7 in accordance with the vertical synchronization signalVss is sequentially executed in increments of rows (time t01).Accordingly, image data corresponding to the exposure period betweentime W0 and time t01 is acquired. The image data acquired at this timeis temporarily saved in memory of the image formation circuit 130 (seeFIG. 7), for example. In this example, resetting of the pixels belongingto the rows of row R0 through row R7 is performed again after readout ofthe pixel signals.

After executing the second reset, the voltage applied to the opposingelectrode 12 is switched to voltage Ve2, thereby starting the secondexposure period of all pixels in the pixel array PA in common. Thesecond exposure period ends by the voltage supply circuit 32 switchingthe voltage applied to the opposing electrode 12 to voltage V3 again.After the second exposure period has ended, readout of pixel signals ofpixels belonging to the rows of row R0 through row R7 is sequentiallyexecuted in increments of rows (time t02), whereby image datacorresponding to the second exposure period is acquired. The point ofthe image data acquired at this time being temporarily saved in memoryof the image formation circuit 130 for example, and the point ofresetting of the pixels belonging to the rows of row R0 through row R7being performed again after readout of the pixel signals, are the sameas acquisition of image data corresponding to the first exposure period.

Thereafter, the same operations are repeated for a desired number oftimes. This yields multiple sets of image data corresponding to theexposure periods. The image formation circuit 130 overlays thesemultiple sets of image data, thereby forming a multiple-exposure image.

Voltages of mutually different magnitudes may be supplied from thevoltage supply circuit 32 to the opposing electrode 12 for each of theexposure periods, during the acquisition of multiple sets of image datato form a multiple-exposure image, as illustrated in FIG. 8. The voltagesupply circuit 32 applies voltages Ve1, Ve2, and Ve3, to the opposingelectrode 12 over the multiple exposure periods in the exampleillustrated in FIG. 8. Ve1<Ve2<Ve3 is satisfied here. In amultiple-exposure image, an image of a subject that moves during oneframe period appears at different positions in the image. Each of theimages of the moving body appearing in the multiple-exposure image canbe imparted with change in display properties by changing the biasvoltage applied to the photoelectric conversion layer 15 for eachexposure period, as in the example described here. For example, thelightness may be changed among each of the images of the moving bodyappearing in the multiple-exposure image. Display attributes changed bychange in bias voltage for each exposure period typically are at leastone of lightness and color (hue or chroma).

FIG. 9 illustrate together an exemplary multiple-exposure image acquiredby the imaging system 100S, and images each including one image of amoving body that have been extracted in time series from themultiple-exposure image. FIG. 9 is an example of five exposure periodsbeing included in one frame period.

In a multiple-exposure image obtained by changing the bias voltageapplied to the photoelectric conversion layer 15 for each exposureperiod, as illustrated to the left side in FIG. 9, the displayattributes are different for each of the images of the moving body.Accordingly, a string of multiple images indicating the way in which themoving body has moved can be constructed from the multiple-exposureimage, as illustrated to the right side in FIG. 9. Accordingly,overlaying multiple sets of image data obtained by changing bias voltageduring the exposure periods to form a multiple-exposure image enablesinformation regarding the way in which the moving body has moved duringthe one frame period (path, change in speed, etc.) to be included in themultiple-exposure image. According to this photography method, increasein the amount of data can be suppressed as a case of sending multiplesets of image data corresponding to each of the exposure periods. Notethat change in voltage supplied by the voltage supply circuit 32 duringthe exposure periods may be monotonous increase as illustrated in FIG.8, monotonous decrease, or random.

An image of an identifier indicating temporal change of the position ofthe moving body may be superimposed on the multiple-exposure image, asillustrated in FIGS. 10 and 11. In the example illustrated in FIG. 10,an arrow connecting the centers of the multiple images of the movingbody is superimposed as the identifier indicating temporal change of theposition of the moving body. In the example illustrated in FIG. 11,numerals are superimposed as the identifier indicating temporal changeof the position of the moving body. Images of the moving body includedin the multiple-exposure image exhibit display properties correspondingto the exposure periods. Accordingly, analyzing the display attributesof the images of the moving body included in the multiple-exposure imageenables identifiers to be provided after formation of themultiple-exposure image. Text, symbols, or the like may be used asidentifiers instead of numerals. Superimposing of the identifier imagemay be executed by the image formation circuit 130.

Signal charges accumulated in the charge accumulation region are readout in accordance with each exposure period in the example describedwith reference to FIG. 8. However, an arrangement may be made whereexposure is performed multiple times, and signal charges accumulated inthe charge accumulation region over one entire frame period are read outto form a multiple-exposure image.

FIG. 12 is a diagram for describing another example of forming amultiple-exposure image. In the example illustrated in FIG. 12, first,resetting of pixels belonging to the rows of row R0 through row R7 andreadout of pixel signals in accordance with the vertical synchronizationsignal Vss is sequentially executed in increments of rows (time t00).Next, voltage Ve1 is applied to the opposing electrode 12, therebyexecuting the first exposure. After the first exposure period, noreadout of pixel signals of the pixels is performed, and voltage Ve2(Ve2>Ve1 here) is applied to the opposing electrode 12, therebyexecuting the second exposure. Accordingly, signal charges correspondingto the second exposure period are further accumulated in the chargeaccumulation region, in addition to the signal charges alreadyaccumulated therein. Such accumulation of signal charges is executed apredetermined number of times, while changing the magnitude of voltageapplied to the opposing electrode 12 during the exposure periods. Thenumber of exposures is five times in this example, with a voltage Ve5that is different from both Ve1 and Ve2 (Ve1<Ve2< . . . Ve5) beingapplied to the opposing electrode 12 in the fifth exposure period.

After the fifth exposure period has ended, readout of pixel signals isexecuted based on the vertical synchronization signal Vss (time t04).That is to say, readout of the total signal charges accumulated overmultiple exposure periods from the signal detection circuit 14 isperformed once during one frame period. In this way, the image formationcircuit 130 may form a multiple-exposure image based on conclusivelyacquired pixel signals, instead of compositing multiple sets of imagedata corresponding to each exposure period.

The image formation circuit 130 is not restricted to a processingcircuit dedicated to forming multiple-exposure images. Forming ofmultiple-exposure images may be realized by a combination of ageneral-purpose processing circuit and a program describing processingfor forming multiple-exposure images. This program may be stored inmemory of the image formation circuit 130, memory of the systemcontroller 120, or the like.

Modifications of Imaging Device

Referencing FIG. 2 again, a global shutter is realized in the embodimentof the present disclosure by different voltages being applied to theopposing electrode 12 between exposure periods and non-exposure periods,as described earlier. In a non-exposure period, the voltage supplycircuit 32 (see FIG. 1) supplies voltage such that the bias voltageapplied to the photoelectric conversion layer 15 falls within theabove-described third voltage range, to the opposing electrode 12 viathe sensitivity control line 42. On the other hand, the potential of thepixel electrode 11 during a non-exposure period is decided by the resetvoltage Vr supplied to the charge accumulation region that partiallyincludes the pixel electrode 11 and impurity region 28 d. As describedearlier, the reset voltage Vr is supplied to the charge accumulationregion via the reset transistor 28 that has the impurity region 28 d asits drain region (or source region). The reset transistor 28 hasfunctions of switching between supply and cutoff of the reset voltage Vrto the charge accumulation region.

In the configuration exemplarily illustrated in FIG. 2, the resetvoltage Vr is supplied from the reset voltage source 34 (see FIG. 1) tothe impurity region 28 s that is the source region (or drain region) ofthe reset transistor 28. The reset voltage source 34 and voltage supplycircuit 32 may be commonalized. Note however, that it is advantageousthat the voltage supply circuit 32 and reset voltage source 34 canindependently supply voltages of different magnitudes as describedlater.

FIG. 13 is a timing chart for describing exemplary operations of thereset voltage source 34 during the reset period. The topmost graph inFIG. 13 illustrates an example of temporal change of voltage Vb appliedfrom the voltage supply circuit 32 to the opposing electrode 12, and thesecond graph illustrates change in voltage level Vrst at the resetcontrol line 48 connected to the gate of the reset transistor 28. Thegraph third from the top illustrates temporal change of potential ϕfd ofthe charge accumulation region. The temporal change of the potential ϕfdcan be said to be representing temporal change of the potential of thepixel electrode 11. Temporal change of potential ϕ at the opposingelectrode 12 with the potential of the pixel electrode 11 as areference, is illustrated below the graph illustrating the temporalchange of potential ϕfd.

Voltage Vc applied to the opposing electrode 12 in a signal readoutperiod including a reset period therein is typically constant, asillustrated in the graph of voltage Vb in FIG. 13. When the voltage ofthe reset control line 48 becomes high level in this state, thepotential ϕfd of the charge accumulation region is reset to Vr by theapplication of the reset voltage Vr via the reset transistor 28.Accordingly, it would seem that if Vc=Vr is satisfied, which is to saythat if the same voltage as the voltage Vc applied to the opposingelectrode 12 is used as the reset voltage Vr, the potential differencebetween the pixel electrode 11 and opposing electrode 12 after resettingwould be expected to be 0.

However, in reality, when the voltage of the reset control line 48 isset to low level and the reset transistor 28 turns off, the potentialϕfd of the charge accumulation region changes due to coupling betweenthe charge accumulation region and the reset transistor 28. In thisexample, the potential ϕfd of the charge accumulation region drops by ΔV(ΔV>0) due to turning-off of the reset transistor 28. Accordingly, whenthe voltage Vc applied to the opposing electrode 12 during the signalreadout period is simply set to be the same as the reset voltage Vr, thepotential difference between the pixel electrode 11 and opposingelectrode 12 after resetting may be outside of the third voltage range.A situation where the potential difference between the pixel electrode11 and opposing electrode 12 after resetting is outside of the thirdvoltage range will result in parasitic sensitivity.

Accordingly, a voltage greater than the voltage Vc applied to theopposing electrode 12 during the signal readout period may be used asthe reset voltage Vr. For example, using a voltage where ΔV is added tothe voltage Vc applied to the opposing electrode 12 as the reset voltageVr, taking into consideration the voltage drop at the chargeaccumulation region due to coupling, enables the potential differencebetween the pixel electrode 11 and opposing electrode 12 after resettingto be brought nearer to 0, and sensitivity due to electric coupling tobe cancelled out.

The specific value of ΔV depends primarily on the characteristics of thereset transistor 28 (typically the parasitic capacitance between sourceand gate), and the value can be known beforehand. For example, ΔV may bemeasured before shipping a product, and the obtained ΔV may be writtento memory (e.g., read-only memory (ROM)) connected to the systemcontroller 120 (see FIG. 7), for example. The system controller 120 cancorrect the magnitude of the reset voltage Vr supplied from the resetvoltage source 34 based on the value of ΔV, by referencing the ΔV storedin memory. Alternatively, the circuit configuration of the reset voltagesource 34 may be adjusted in accordance with the value of ΔV, so thatthe output voltage is the desired voltage. Voltage supplied from thevoltage supply circuit 32 to the opposing electrode 12 may also becorrected, either instead of correcting the reset voltage Vr suppliedfrom the reset voltage source 34 or along with correction of the resetvoltage Vr. Note however, that correction of the reset voltage Vr ismore advantageous than correction of voltage supplied from the voltagesupply circuit 32 to the opposing electrode 12, with regard to the pointthat correction can be performed for each pixel. Such calibration ofreset voltage Vr (and/or voltage supplied to the opposing electrode 12)may be executed before shipping of the imaging device 100, or may beexecuted by the user of the imaging device 100.

In a case where the reset transistor 28 is a P-channel transistor, thepotential ϕfd of the charge accumulation region rises by ΔV due toturning-off of the reset transistor 28, as illustrated in FIG. 14.Accordingly, in a case of using a P-channel transistor for the resettransistor 28, a voltage smaller than the voltage Vc applied to theopposing electrode 12 in the signal readout period may be used as thereset voltage Vr.

In the example illustrated in FIGS. 13 and 14, the potential ϕ of theopposing electrode 12, with the potential of the pixel electrode 11 as areference, is outside of the third voltage range, in the period beforethe reset period. The voltage Vb applied from the voltage supply circuit32 to the opposing electrode 12 does not need to be a voltage where thepotential difference between the pixel electrode 11 and opposingelectrode 12 is within the third voltage range through the entirenon-exposure period, as illustrated in these examples. The potential ϕof the opposing electrode 12 with the potential of the pixel electrode11 as a reference may be outside of the third voltage range beforeresetting the pixels.

Thus, using a corrected voltage as the reset voltage Vr enablesoccurrence of parasitic sensitivity due to electrical coupling to besuppressed. If the correction value used at this time is too great, agreat potential difference will occur between the pixel electrode 11 andthe opposing electrode 12, and there is a possibility of charges in thecharge accumulation region flowing to the opposing electrode 12 via thephotoelectric conversion layer 15. In other words, this is a risk ofbackflow of charge via the photoelectric conversion layer 15.Accordingly, it is advantageous that the absolute value of thedifference between the reset voltage Vr and the voltage Vc that thevoltage supply circuit 32 applies to the opposing electrode 12 issmaller than the breakdown voltage of the photoelectric conversion layer15. For example, in a case where the reset transistor 28 is an N-channeltransistor, it is advantageous that the reset voltage Vr does not exceedthe voltage Vc. The breakdown voltage of the photoelectric conversionlayer 15 can be defined as a voltage at which the photoelectricconversion layer 15 loses its function due to charges in the chargeaccumulation region flowing from the pixel electrode 11 to the opposingelectrode 12 via the photoelectric conversion layer 15. Alternatively,it is advantageous that the absolute value of the difference between thereset voltage Vr and the voltage Vc that the voltage supply circuit 32applies to the opposing electrode 12 is smaller than the input voltageto the signal detection circuit 14 (typically VDD).

FIG. 15 illustrates a modification of the imaging device 100. In theconfiguration exemplarily illustrated in FIG. 15, the semiconductorsubstrate 20 has a substrate voltage supply circuit 35 that supplies apredetermined substrate voltage Vs. The substrate voltage Vs suppliedfrom the substrate voltage supply circuit 35 is voltage that isdifferent from 0 V.

Setting the reset voltage Vr to a voltage near 0 V enables the voltageVc applied from the voltage supply circuit 32 to the opposing electrode12 to be 0 V, i.e., enables the opposing electrode 12 to serve as aground, so the circuit configuration of the imaging device 100 can befurther simplified. However, if the reset voltage Vr is 0 V for example,the signal detection transistor 24 will not function as asource-follower, so signal voltage cannot be read out.

In the configuration exemplified in FIG. 15, substrate voltage Vs thatis different from 0 V is applied to the semiconductor substrate 20. Forexample, a negative voltage is applied to the semiconductor substrate 20as substrate voltage Vs, thereby shifting the substrate potential.Shifting the substrate potential enables both suppression of darkcurrent and linearity at the signal detection circuit 14 to be realized,even in a case where the reset voltage Vr, and the voltage Vc appliedfrom the voltage supply circuit 32 to the opposing electrode 12, are 0V. The substrate voltage supply circuit 35 may be commonalized with theabove-described voltage supply circuit 32 and/or reset voltage source34. An arrangement where the voltage Vc that is applied from the voltagesupply circuit 32 to the opposing electrode 12 is positive voltageenables the same advances as a case where the reset voltage Vr andvoltage Vc are set to 0 V to be obtained, while avoiding application ofnegative voltage to the semiconductor substrate 20.

As described above, according to the embodiment of the presentdisclosure, controlling the voltage applied to the opposing electrode 12enables accumulation and storage of charges to the charge accumulationregion to be controlled. Thus, a global shutter function can be realizedwith a simpler device structure.

Various other modifications besides the above-described examples can bemade to the imaging device according to the embodiment of the presentdisclosure. For example, global shutter driving and rolling shutterdriving may be switched in accordance with the subject. In rollingshutter driving, the voltage that the voltage supply circuit 32 appliesto the opposing electrode 12 can be fixed to voltage Ve for bothexposure period and non-exposure period. The exposure period can bestipulated here by the period from the time of resetting the chargeaccumulation node 41 to the time of readout of signals.

Each of the above-described signal detection transistor 24, addresstransistor 26, and reset transistor 28, may be N-channel MOS, orP-channel MOS. There is no need for all of these to be unified toN-channel MOS or P-channel MOS. Bipolar transistors may be used as thesignal detection transistor 24 and/or address transistor 26, besidesfield-effect transistors.

The imaging device according to the present disclosure is applicable toimage sensors and the like, for example. The imaging device according tothe present disclosure can be used in medical cameras, robot cameras,security cameras, vehicle onboard-use cameras, and so forth. Vehicleonboard-use cameras can be used as input as to a control device, forsafe traveling of the vehicle. Alternatively, vehicle onboard-usecameras may be used for support of an operator, for safe traveling ofthe vehicle.

What is claimed is:
 1. An imaging device comprising pixels eachincluding: a photoelectric converter including a first electrode, asecond electrode, a photoelectric conversion layer between the firstelectrode and the second electrode; a charge accumulation regionelectrically connected to the first electrode; and a signal detectioncircuit electrically connected to the charge accumulation region; and avoltage supply circuit electrically connected to the second electrode,wherein the photoelectric converter is adapted to be applied with avoltage between the first electrode and the second electrode, and thephotoelectric converter has a characteristic, responsive to the voltagewithin a range from a first voltage to a second voltage, showing that adensity of current passing between the first electrode and the secondelectrode when light is incident on the photoelectric conversion layerbecomes substantially equal to that when no light is incident on thephotoelectric conversion layer, a difference between the first voltageand the second voltage is 0.5 V or more, and the voltage supply circuitsupplies a voltage between the first voltage and the second voltage tothe second electrode in a non-exposure period.
 2. The imaging deviceaccording to claim 1, wherein the second electrodes of the pixels areelectrically connected to each other.
 3. The imaging device according toclaim 1, wherein the first voltage is greater than −1 V, and the secondvoltage is smaller than +1 V.
 4. The imaging device according to claim1, wherein the photoelectric converter has a characteristic, responsiveto the voltage within the range from the first voltage to the secondvoltage, showing that the density of the current when light is incidenton the photoelectric conversion layer is substantially constant.
 5. Theimaging device according to claim 1, wherein the photoelectric converterhas a characteristic, responsive to the voltage within the range fromthe first voltage to the second voltage, showing that an absolute valueof the density of the current passing between the first electrode andthe second electrode when light is incident on the photoelectricconversion layer is 100 μA/cm² or less.